**4.2 Physicochemical changes**

Water is one of the main constituents of grape berries, and significant amounts are required for their full growth and development [43]. At maturity, grape berries have a water content of around 75–80% of their fresh weight [49].

Throughout berry development, water losses occur mainly due to transpiration, and this intensity depends on climatic conditions and changes during berry development [49]. Most of the water required by the fruit is supplied by the xylem until *Véraison*, but after this period the xylem vessels present in the berry are blocked and water transport is carried by the phloem, the main supplier of water to the fruit [44, 49].

Sugars result from the photosynthesis process carried out in the green organs of the vine, migrating to the various parts of the plant in the form of sucrose [49]. Until the beginning of the *Véraison*, sugars are consumed in cell growth, but also by migrating to the fruit for the growth and maturation of the seeds [44]. Sugars are the basis for several compounds, such as organic acids and amino acids, synthesized and found in the fruit [43].

Sucrose, a sugar predominantly transported in the phloem, is formed by the union of a glucose and a fructose molecule. When the sucrose is in the berry it is hydrolyzed, forming again the referred hexoses (fructose and glucose), existing in the pulp [43, 44].

At harvest, the amounts of glucose and fructose are approximately identical, varying between 8 and 12% of the fresh weight of the fruits, and after maturity there is a tendency for fructose to predominate [43]. Sucrose and other sugars are present in the fruit, but in very small amounts [43].

The main organic acids present in grape berries are tartaric, malic and citric acids, with the first two representing more than 90% of the total acids in the berry [43–45]. Tartaric acid is a secondary product of sugar metabolism and its content increases during herbaceous growth due to intense cell multiplication. Regarding malic acid, it is an intermediate of sugar metabolism and during herbaceous growth the sugar produced gives rise to this acid that is stored in the vacuoles of the pulp cells [45]. Tartaric acid is biosynthesized before *Véraison*, so the amount per berry remains stable, while malic acid is biosynthesized before *Véraison*, but also during ripening, and is degraded through respiration, which consequently leads to a decrease in its amount per berry [46, 49].

During *Véraison* and the ripening period, the berry volume increases and the membrane tension of the vacuoles in the pulp cells starts to decrease, which leads to the degradation of malic acid [43, 46].

Phenolic compounds, also called polyphenols, are organic compounds that result from the secondary metabolism of plants and are biosynthesized through the shikimic acid cycle. They are defined as substances that have an aromatic ring consisting of six carbon atoms with one or more hydroxyl groups or derivatives of this basic structure [49]. The phenolic content of plant-based food depends on intrinsic factors such as genus, species and variety and extrinsic factors such as agronomic and environmental conditions, ripening process, and storage conditions. Phenolic compounds are present in the berry since its formation, resulting from the catabolism of sugars [44]. They are synthesized in the berry, with different amounts, proportion and types in the skin, pulp, and seeds, and can vary significantly among cultivars [44, 50]. Regarding the total phenolic compounds present in the berry, it is known that in the skin the total extractable phenolic compounds are between 28 and 35%, the pulp presents values below 10% and the seeds between 60 and 70% [50]. Grape is one of the major sources of phenolic compounds in the human diet, the main classes of phenolics compounds in grapes are flavan-3-ols, tannins, anthocyanins, flavonols, hydroxycinnamic acids, hydroxybenzoic acids and stilbenes [49]. These compounds are of great interest since they have high nutritional value and protective function against diseases caused by oxidative damage, such as heart disease, stroke and cancer [49]. White grapes, when compared to red grapes, have lower total phenolics contents, partially because they do not synthesize anthocyanins in significant amounts [44]. These compounds can act as antioxidants in several ways, namely by scavenging free radicals, scavenging oxygen radicals and as chelators of metal ions [50]. Moreover, they play an important role in grape quality, since they inhibit lipid oxidation and participate in the processes responsible for color, astringency and aroma, inhibit lipid oxidation and fungal proliferation [50].

Mineral elements naturally originate in the soil and their accumulation in grape berries is accomplished via the xylem, except for potassium which accumulates via phloem [51]. These elements constitute between 0.2 and 0.6% of the fresh weight of the berry [52]. During berry growth, the accumulation of large amounts of nitrogen, calcium, phosphorus, and magnesium occurs, with the main mineral being potassium [49]. The accumulation of nitrogen and potassium is carried out before and after *Véraison*, while the accumulation of calcium, phosphorus and magnesium

*Table Grapes: There Is More to Vitiviniculture than Wine… DOI: http://dx.doi.org/10.5772/intechopen.99986*

is preferentially carried out after *Véraison* [46]. The distribution of mineral elements between the epidermis and pulp and their accumulation in the berry varies depending on factors such as variety, climatic conditions, and water availability [46]. The accelerated berry transpiration could be associated with higher fruit mineral nutrient content [49]. Mineral elements are highly important for human nutrition because they are not synthesized by our body, which has led to an increasing interest in studies on the constitution of fruits and vegetables in this type of elements.

There are different definitions for food texture, and it can be evaluated through sensory analysis and/or instrumental methods, which are related to the evaluation of food structure and the determination of its chemical composition. Textural attributes vary during the pre- and postharvest period, being affected by ripening stage, plant nutrition, water stress, storage temperature and relative humidity [53].

The fruit texture is dependent on the biomolecules involved in the cellular structure of the cell walls being the changes mostly attributed to changes in the composition and structure of cell wall polysaccharides [54].

With the initiating changes in fruit texture, there are modifications in the chemistry of the middle lamella and primary cell wall components (pectins, celluloses, and hemicelluloses) that accelerate the loss of fruit firmness [55, 56]. Studies conducted during storage period of grapes suggest that a reduction of cell wall pectins and hemicelluloses occurs, since during fruit ripening these undergo solubilization and depolymerization, which contributes to cell wall disintegration [56, 57]. Moreover, softening has also been associated with the flow of carbohydrates and osmotically active nutrients to the fruit due to competition for the accumulated reserves and the phytohormonal-caused differential movement of solutes [38].

According to Ejsmentewicz et al. [56], homogalacturonan (HG) is proposed as one of the main components of the cell wall, involved in the texture changes of fruits.

The importance of texture evaluation is due to the knowledge of these textural changes during ripening, and storage and with the differences found among varieties, being a quality attribute valued by consumers in table grapes.

The rheological behavior of foods is related to the deformation, disintegration, and flow when a force is applied, and the response can be evaluated as a function of force, time, and deformation. According to Abbott [58], fruits have a viscoelastic behavior when subjected to a load, so the force, time and deformation (intensity, duration and speed of the load) determine their rheological behavior.

In table grapes, the instrumental determination of the consistency of the berry epidermis and the compactness of the pulp, provides relevant information about the acceptability of the product by the consumer [59]. Grape berry texture is one of the most important quality parameters affecting the consumption of this fruit [56, 58].

According to Rolle et al. [60], from the point of view of consumer texture of table grape berry includes different attributes, mainly hardness (firmness), elasticity, shape, and sensations in the mouth during chewing.

The texture analysis is a rapid, and low-cost analytical technique, that can be applied in viticulture and enology as a routine monitoring tool for the grape quality. Previous studies have indicated that the grape texture is linked to cultivar and growing location, reflecting a terroir influence on grape quality [61, 62], and instrumental texture parameters were used to investigate the effects of vineyard practices [38, 50].

The color of the grape skin or exocarp is classified as green-yellow, pink, red, red-gray, violet-dark red, blue-black and red-black [63]. This attribute can be easily assessed instrumentally in color spaces, the most commonly used being CIELab, in which the color is defined by the coordinates L\*, a\* and b\*.
